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References: <e3b48104-3efb-1896-0d46-792419f49a75@virtuozzo.com> <20180731170957.o4vhopmzgedpo5sh@breakpoint.cc>
In-Reply-To: <20180731170957.o4vhopmzgedpo5sh@breakpoint.cc>
From: Eric Dumazet <edumazet@google.com>
Date: Tue, 31 Jul 2018 10:32:39 -0700
Message-ID: <CANn89iLKwJ5oDfXu8M9z9_Xh0FKHFo961c_ocdUmHQc3Onmykw@mail.gmail.com>
Subject: Re: SLAB_TYPESAFE_BY_RCU without constructors (was Re: [PATCH v4
 13/17] khwasan: add hooks implementation)
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Sender: owner-linux-mm@kvack.org
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To: Florian Westphal <fw@strlen.de>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>, Theodore Ts'o <tytso@mit.edu>, jack@suse.com, linux-ext4@vger.kernel.org, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Pablo Neira Ayuso <pablo@netfilter.org>, Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>, David Miller <davem@davemloft.net>, netfilter-devel@vger.kernel.org, coreteam@netfilter.org, netdev <netdev@vger.kernel.org>, Gerrit Renker <gerrit@erg.abdn.ac.uk>, dccp@vger.kernel.org, jani.nikula@linux.intel.com, joonas.lahtinen@linux.intel.com, rodrigo.vivi@intel.com, airlied@linux.ie, intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>, Ursula Braun <ubraun@linux.ibm.com>, linux-s390@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>, Dmitry Vyukov <dvyukov@google.com>, Christoph Lameter <cl@linux.com>, Andrew Morton <akpm@linux-foundation.org>, linux-mm <linux-mm@kvack.org>, Andrey Konovalov <andreyknvl@google.com>, Linus Torvalds <torvalds@linux-foundation.org>

On Tue, Jul 31, 2018 at 10:10 AM Florian Westphal <fw@strlen.de> wrote:
>
> Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> > Guys, it seems that we have a lot of code using SLAB_TYPESAFE_BY_RCU cache without constructor.
> > I think it's nearly impossible to use that combination without having bugs.
> > It's either you don't really need the SLAB_TYPESAFE_BY_RCU, or you need to have a constructor in kmem_cache.
> >
> > Could you guys, please, verify your code if it's really need SLAB_TYPSAFE or constructor?
> >
> > E.g. the netlink code look extremely suspicious:
> >
> >       /*
> >        * Do not use kmem_cache_zalloc(), as this cache uses
> >        * SLAB_TYPESAFE_BY_RCU.
> >        */
> >       ct = kmem_cache_alloc(nf_conntrack_cachep, gfp);
> >       if (ct == NULL)
> >               goto out;
> >
> >       spin_lock_init(&ct->lock);
> >
> > If nf_conntrack_cachep objects really used in rcu typesafe manner, than 'ct' returned by kmem_cache_alloc might still be
> > in use by another cpu. So we just reinitialize spin_lock used by someone else?
>
> That would be a bug, nf_conn objects are reference counted.
>
> spinlock can only be used after object had its refcount incremented.
>
> lookup operation on nf_conn object:
> 1. compare keys
> 2. attempt to obtain refcount (using _not_zero version)
> 3. compare keys again after refcount was obtained
>
> if any of that fails, nf_conn candidate is skipped.


Yes, the key here is the refcount, this is only what we need to clear
after kmem_cache_alloc()

By definition, if an object is being freed/reallocated, the refcount
should be already 0, and clearing it again is a NOP.